Heat shock-mediated cell cycle blockage and G1 cyclin expression in the yeast Saccharomyces cerevisiae

The heat shock response in yeast: differential regulations and contributions of the Msn2p/Msn4p and Hsf1p regulons

The heat shock transcription factor Hsf1p and the stress-responsive transcription factors Msn2p and Msn4p are activated by heat shock in the yeast Saccharomyces cerevisiae. Their respective contributions to heat shock protein induction have been analysed by comparison of mutants and wild-type strains using [35S]-methionine labelling and two-dimensional gel electrophoresis. Among 52 proteins induced by a shift from 25 degrees C to 38 degrees C, half of them were found to be dependent upon Msn2p and/or Msn4p (including mostly antioxidants and enzymes involved in carbon metabolism), while the other half (including mostly chaperones and associated proteins) were dependent upon Hsf1p. The two sets of proteins overlapped only slightly. Three proteins were induced independently of these transcription factors, suggesting the involvement of other transcription factor(s). The Ras/cAMP/PKA signalling pathway cAMP had a negative effect on the induction of the Msn2p/Msn4p regulon, but did not affect the Hsf1p regulon. Thus, the two types of transcription factor are regulated differently and control two sets of functionally distinct proteins, suggesting two different physiological roles in the heat shock cellular response.

Heat-induced cell cycle arrest of Saccharomyces cerevisiae: involvement of the RAD6/UBC2 and WSC2 genes in its reversal

The Saccharomyces cerevisiae RAD6 (UBC2 ) gene encodes a ubiquitin-conjugating enzyme that is involved in a wide range of cellular processes including DNA repair, sporulation and N-end rule protein degradation. Under mild heat stress conditions (37-38 degrees C) rad6 null and rad6-149 mutant cells are unable to grow. The molecular basis for this failure to grow is unknown. Here we show that the heat sensitivity of rad6 mutants is not due to cell death but to an inability to progress in the cell cycle. The temperature-induced cell cycle arrest of these mutants is due to a block in a branch of the RAD6 pathway distinct from the DNA repair and the N-end rule protein degradation pathways. Wild-type cells heated to 38 degrees C arrest transiently in the late G1 phase and then resume growth. At 38 degrees C rad6 mutant cells arrest in late G1 but, unlike wild-type cells, are unable to resume cell cycle progression. In both wild-type and in rad6 mutant cells, CLN1 and CLN2 transcript levels fall sharply upon temperature increase. In wild-type cells levels of these transcripts recover rapidly, whereas in the rad6 mutant they recover slowly. As rad6 cells remain arrested even after CLN1 and CLN2 mRNAs regain their preheat stress levels, factors additional to reduced G1 cyclin gene expression must cause the temperature-induced cell cycle block of the mutant. To identify genes involved in the relief of the cell cycle arrest under heat stress, we screened a multicopy yeast genomic library for clones that restore the growth of the rad6-149 mutant. A plasmid was isolated carrying the WSC2 gene, which is closely related to WSC1/SLG1/HCS77, a putative membrane heat sensor. Overexpression of WSC2 reverses the heat-induced cell cycle arrest of rad6-149 but not of rad6 null mutants. Taken together the findings point to the existence of an unidentified heat stress-activated cell cycle checkpoint pathway, which is antagonized by Rad6p by a mechanism also involving Wsc2p.

Finding DNA regulatory motifs within unaligned noncoding sequences clustered by whole-genome mRNA quantitation

Whole-genome mRNA quantitation can be used to identify the genes that are most responsive to environmental or genotypic change. By searching for mutually similar DNA elements among the upstream non-coding DNA sequences of these genes, we can identify candidate regulatory motifs and corresponding candidate sets of coregulated genes. We have tested this strategy by applying it to three extensively studied regulatory systems in the yeast Saccharomyces cerevisiae: galactose response, heat shock, and mating type. Galactose-response data yielded the known binding site of Gal4, and six of nine genes known to be induced by galactose. Heat shock data yielded the cell-cycle activation motif, which is known to mediate cell-cycle dependent activation, and a set of genes coding for all four nucleosomal proteins. Mating type alpha and a data yielded all of the four relevant DNA motifs and most of the known a- and alpha-specific genes.

The CLN gene family: central regulators of cell cycle Start in budding yeast

The Start transition in the budding yeast cell cycle is the point of most physiological regulation of cell cycle commitment. This transition is controlled by the CLN1,2,3 gene family. We review what is known about the regulation, inter-regulation and function of these genes in controlling the Start transition.

Transcriptional remodeling and G1 arrest in dioxygen stress in Saccharomyces cerevisiae

Saccharomyces cerevisiae, which lack a functional SOD1 gene, encoding the cytosolic Cu,Zn-superoxide dismutase (SOD1), exhibit a variety of metabolic defects in aerobic but not in anaerobic growth. We test here the hypothesis that some of these defects may be due to specific transcriptional changes programmed for cell survival under dioxygen stress. Analysis of the budding pattern and generation time showed that the slower proliferation of an sod1Delta mutant strain under air was due to an increase from 42 to 89 min spent in the G1 phase of the cell cycle. This delay in G1 was not due to an overall decline in biosynthetic activity since total protein and mRNA synthesis was not reduced even under 100% O2. However, rRNA synthesis was strongly decreased, e.g. by 80% in the mutant under 100% O2 (in comparison to N2). Under these conditions, the mutant permanently arrested in G1; this arrest was due to an inhibition of the Start function that prepares yeast for S phase. This Start arrest was due to an inhibition of transcription of the autoregulated G1 cyclins, CLN1 and CLN2; the transcription of the constitutive G1 cyclin, CLN3, was unaffected by the stress. Expression of a hyperstable Cln3 prevented the G1 arrest, indicating that it was due solely to the inhibition of cell cycle-dependent cyclin expression. This remodeling of transcription in oxidative stress was seen also in the inhibition of glucose derepression of SUC2 expression. In contrast, the signaling and activation of mating pheromone (FUS1) and copper-responsive (CUP1) promoter activity were not affected by dioxygen stress, while genes encoding other anti-oxidant enzymes (SOD2, CTT1 and CTA1) were strongly induced. The UBI loci, encoding ubiquitin, were particularly good examples of this pattern of negative and positive transcriptional response to the stress. UBI1-UBI3 expression was repressed in the mutant under 100% O2, while expression of UBI4 was strongly induced. The data demonstrate that extensive remodeling of transcription occurs in yeast under a strong dioxygen stress. This remodeling results in a pattern of expression of gene products needed for defense and repair, and suppression of activities associated with normal proliferative growth.

Hyperthermia and paraquat-induced G1 arrest in the yeast Saccharomyces cerevisiae is independent of the RAD9 gene

Mutants of the RAD9 gene of Saccharomyces cerevisiae are defective in cell cycle checkpoint arrest in G1 and G2 after treatment with DNA-damaging agents. It is demonstrated that the pronounced G1 arrest observed in yeast after hyperthermia treatment or exposure to paraquat-generated superoxide radicals does not depend on a functional RAD9 gene. For both types of treatments, the mechanism of cell cycle arrest must be considered different from the activation of the DNA damage checkpoint response.

Starting the cell cycle: what's the point?

Early work on regulation of the budding yeast cell cycle defined a critical regulatory step called 'Start', considered to represent cell cycle commitment. Recent work has defined the probable molecular basis of Start to be activation of Cln-Cdc28 protein kinase complexes. Cln-Cdc28 kinases may directly regulate many cell cycle processes, including some classically considered to be 'post-Start'. Specialization of function among the three genetically redundant CLN genes is becoming apparent.

The Yak1 protein kinase of Saccharomyces cerevisiae moderates thermotolerance and inhibits growth by an Sch9 protein kinase-independent mechanism

The growth defect associated with the loss of yeast A kinase activity can be alleviated by the overexpression or deletion of two other kinases, Sch9 and Yak1, respectively. Using tests of epistasis, we have shown that Sch9 and Yak1 define separate signaling pathways and must, therefore, suppress the A kinase defect by different mechanisms. Nevertheless, the Yak1 kinase appears to regulate cellular processes that are under A kinase control. For example, acquisition of heat resistance is correlated with Yak1 kinase activity, such that YAK1-overexpressing cells are over 200-fold more resistant than isogenic yak1 strains. These results, for the first time, associate a phenotype, other than suppression of the A kinase growth defect, with the loss of Yak1 activity and argue a broader role for the Yak1 kinase in cell growth.

An impaired RNA polymerase II activity in Saccharomyces cerevisiae causes cell-cycle inhibition at START

Saccharomyces cerevisiae cells harboring the temperature-sensitive mutation rpo21-4, in the gene encoding the largest subunit of RNA polymerase II, were shown to be partially impaired for cell-cycle progress at a permissive temperature, and to become permanently blocked at the cell-cycle regulatory step, START, at a restrictive temperature. The rpo21-4 mutation was lethal in combination with cdc28 mutations in the p34 protein kinase gene required for START. Transcripts of the CLN1 and CLN2 genes, encoding G1-cyclin proteins that, along with p34, are necessary for START, were decreased in abundance by the rpo21-4 mutation at a restrictive temperature. Increased G1-cyclin production, by expression of the CLN1 or CLN2 genes from a heterologous GAL promoter, overcame the rpo21-4-mediated START inhibition, but such mutant cells nevertheless remained unable to proliferate at a restrictive temperature. These findings reveal that START can be particularly sensitive to an impaired RNA polymerase II function, presumably through effects on G1-cyclin expression.

Molecular events associated with acquisition of heat tolerance by the yeast Saccharomyces cerevisiae

The heat shock response is an inducible protective system of all living cells. It simultaneously induces both heat shock proteins and an increased capacity for the cell to withstand potentially lethal temperatures (an increased thermotolerance). This has lead to the suspicion that these two phenomena must be inexorably linked. However, analysis of heat shock protein function in Saccharomyces cerevisiae by molecular genetic techniques has revealed only a minority of the heat shock proteins of this organism having appreciable influences on thermotolerance. Instead, physiological perturbations and the accumulation of trehalose with heat stress may be more important in the development of thermotolerance during a preconditioning heat shock. Vegetative S. cerevisiae also acquires thermotolerance through osmotic dehydration, through treatment with certain chemical agents and when, due to nutrient limitation, it arrests growth in the G1 phase of the cell cycle. There is evidence for the activities of the cAMP-dependent protein kinase and plasma membrane ATPase being very important in thermotolerance determination. Also, intracellular water activity and trehalose probably exert a strong influence over thermotolerance through their effects on stabilisation of membranes and intracellular assemblies. Future investigations should address the unresolved issue of whether the different routes to thermotolerance induction cause a common change to the physical state of the intracellular environment, a change that may result in an increased stabilisation of cellular structures through more stable hydrogen bonding and hydrophobic interactions.